More fickle than ever, advertisers are quitting their agencies in search of sharper ideas and better deals. Result: fear on Madison Avenue.

Avis believes everybody should try harder. When it decided earlier this year that it might want to hire a new advertising firm to handle its $35 million account, the company considered virtually every major agency in the phone book -- 100 of them -- for the job. Once Avis chose six finalists, the agencies poured their energy into the project. Researchers made studies of consumer driving trends. Copywriters crafted catchy new slogans. Creative teams worked up lavish demonstrations of their talent with music, art, sample ads and commercials. Last week Avis announced that after examining the industry's best efforts, it had...